Add Zamba2 architecture adapter unit tests

[abhinav-bellapu]

Description

Adds a focused, model-download-free unit suite for Zamba2ArchitectureAdapter.
This complements the existing slow integration coverage by pinning the adapter's
fast, architecture-specific contracts:

• config flags and standard past_key_values cache routing
• hybrid layer metadata and shared-attention settings
• Mamba intermediate and convolution dimension formulas
• optional block and mixer submodule wiring
• bridge types, Hugging Face module paths, and factory selection

No runtime behavior changes are included.

Related to #1302.

Type of change

• Test coverage

Validation

• uv run --python 3.12 pytest tests/unit/model_bridge/supported_architectures/test_zamba2_adapter.py -q (12 passed)
• make check-format
• uv run --python 3.12 mypy . (316 source files)
• uv build
• Full non-slow unit surface exercised across complementary runs:
  • online: 3,114 passed before a Hugging Face gpt2 API socket stalled
  • offline: 3,452 passed; 10 setup errors were SoLU API calls intentionally blocked by offline mode
  • the previously stalled gpt2 test passes in offline-cache mode

CI is the authoritative combined unit run because its model cache avoids both
local conditions.

Checklist

• The tests explain architecture-specific intent where it is non-obvious
• No documentation change is required for test-only coverage
• My changes generate no new warnings
• I have added tests that cover the intended adapter contracts
• I have not rewritten tests for existing key interfaces